Measuring the thickness of lubricants used in chipless metal processing

Whether rolling, pressing, drawing or stamping, chipless coldforming of sheet metals simply does not work without lubrication. Especially in the automotive industry, the need for ever reduced air resistance and to meet ever more challenging design requirements continues to grow. The result of the forming process depends largely on proper lubrication, which is used, for example, to reduce friction between the forming tool and the workpiece to prevent them from sticking together – a condition that can exert negative impacts on both the workpiece and the expensive forming dies used. The thickness of the lubricant film must therefore meet specifications within a narrow tolerance range.

Besides reducing friction, lubricants help prevent typical process-induced flaws such as cracks, scratches and dents. The oils are applied by roller to the sheets in process and distributed evenly over the entire surface – in sheet metal forming, about 0.5- 2 g/m2. The respective predetermined tolerances must be closely monitored through measurement. The most suitable, non-destructive method uses the principle of beta backscatter, in which beta particles (electrons) are given off by a radionuclide (emitter); some of them are then scattered back by the oil layer and the metal substrate.

The measurement effect consists of the number of backscattered electrons, which in turn is dependent on the mass per unit of the coating and the atomic numbers of the coating and base material. This is the method that has been implemented in the modular coating thickness gauge, FISCHERSCOPE® MMS® PC2, to be used in conjunction with the probe Z15NG and a C14 beta emitter. A three-point support that touches only outside the recognized measurement area ensures a stable, reproducible probe placement and the optimal working distance of approximately 4 mm between probe and measured surface – allowing for the contactless measuring that is essential with these coatings.

Fig.2: Schematic representation of an oil film measurement using the probe Z15NG and the beta backscatter method

Table 1 shows measurement examples of an oil film on two different base materials. The higher standard deviation in the sample “oil/Al/Fe” is caused by variations in the thickness of the aluminium.

oil/Fe

oil/Al/Fe

Oil film

3.8 g/m²

1.9 g/m²

Standard variation

0.07 g/m²

0.3 g/m2

Number of measurements

10

5

Measurement time

5 s

20 s

Tab.1: Measurement with Z15NG probe according to the beta backscatter method

With the FISCHERSCOPE® MMS® PC2 and the probe Z15NG, it is possible to measure the thickness of lubricants on metals to ensure optimal workflows for chipless sheet metal forming. For more information, please contact your local sales representative from FISCHER.